A fabrication process for a semiconductor die begins with a semiconductor wafer on which a large number of semiconductor dice have been formed by doping, masking, deposition of metals, and etching a silicon substrate. Following the fabrication process the wafer is probed and mapped. Wafer probe is performed to test the gross functionality of the dice on the wafer. In addition, the nonfunctional dice are mechanically marked or mapped in software.
In the past semiconductor wafers have been probed utilizing needle probes formed on a probe card. The needle probes provide a temporary electrical connection between test pads on the dice and external test circuitry. The probe card typically includes an insulating substrate, such as a glass filled plastic, and the needle probes are soldered to openings in the substrate in electrical communication with circuit traces on the substrate.
One problem with this type of conventional probe card is that the planarity and vertical position of the needle probes can vary. Also the z-direction location of the test contacts on the wafer can vary. This can cause inaccuracies in the test results because electrical contact with the different test pads can vary. Continued use of the needle probes causes deformation and further misalignment of the needle probes. Probe cards with needle probes are thus expensive to fabricate and expensive to maintain. In addition, needle probes do not adequately compensate for vertical misalignment between the test pads on the wafer.
Another problem with needle probe cards is that the test pads on the wafer are typically coated with a metal oxide layer. For example, aluminum test pads can be covered with aluminum oxide that forms by oxidation of the underlying metal. This metal oxide is electrically non-conductive and provides a high degree of electrical resistance to the needle probes. In order to ensure accurate test results, the needle probes must penetrate this oxide layer to the underlying metal film. This requires high contact forces which can damage the test pads.
To overcome some of the problems associated with conventional needle probes, membrane probe cards have been developed. U.S. Pat. Nos. 4,891,585; 4,918,383; 4,906,920 and 5,180,977 disclose representative membrane probe cards. Membrane probe cards typically include a membrane formed of a thin and flexible dielectric material such as polyimide. Contact bumps are formed on the membrane in electrical communication with conductive traces. The conductive traces electrically connect to external test circuitry.
In general, membrane probes are able to compensate for vertical misalignment between the test pads on the wafer. In addition, the membrane probe can include a support mechanism that allows the contact bumps to scrub the surface of the test contacts on the wafer to penetrate the oxide layer.
Membrane probes are typically formed by plating a metal layer on a flexible dielectric surface and then patterning and etching the metal layer using photolithographic methods. The contact bumps can also be formed by plating or other deposition process. However, the manufacturing process for membrane probes is complex and expensive. In addition, the support mechanisms for membrane probes are also complicated and can require a large number of moving parts.
Another disadvantage of membrane probes is that large contact forces are required to make a reliable electrical connection between the contact bumps on the probe and the test pads on the wafer. These contact forces include a vertical "overdrive" force and a horizontal "scrubbing" force. These large forces can damage the test pads and the wafer. In addition, the contact bumps and membranes are repeatedly stressed by the large forces. These forces can cause the membrane to lose its resiliency.
In addition, an elastomeric member is typically formed between the membrane probe and the support mechanism to cushion the force applied by the membrane probe to the wafer. This elastomeric member can also be adversely affected by the large contact forces. High temperatures used during the test procedure can also compound this problem. It is well known that elastomeric materials exhibit a compression set or "creep" under stress. High temperatures and large contact forces increase creep in the elastomeric member and in the membrane of the probe.
In view of the foregoing problems associated with conventional probe cards for testing semiconductor dice, it is an object of the present invention to provide an improved probe card for testing semiconductor dice contained on a wafer.
It is another object of the present invention to provide an improved method for forming a probe card for testing semiconductor dice contained on a wafer.
It is yet another object of the present invention to provide an improved method for testing semiconductor dice contained on a wafer using a silicon micromachined probe card.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.